This invention relates to carrier particles for use in dry powder inhalers. More particularly the invention relates to a method of producing such particles, to a dry powder incorporating the particles and to the particles themselves.
Inhalers are well known devices for administering pharmaceutical products to the respiratory tract by inhalation. Inhalers are widely used particularly in the treatment of diseases of the respiratory tract.
There are a number of types of inhaler currently available. The most widely used type is a metered dose inhaler (MDI) which uses a propellant to expel droplets containing the pharmaceutical product to the respiratory tract. Those devices are disadvantageous on environmental grounds as they use CFC propellants.
An alternative device to the MDI is the dry powder inhaler. The delivery of dry powder particles of pharmaceutical products to the respiratory tract presents certain problems. The inhaler should deliver the maximum possible proportion of the active particles expelled to the lungs, including a significant proportion to the lower lung, preferably at the low inhalation capabilities to which some patients, especially asthmatics, are limited. It has been found, however, that, when currently available dry powder inhaler devices are used, in many cases only about 10% of the active particles that leave the device on inhalation are deposited in the lower lung. More efficient dry powder inhalers would give clinical benefits.
The type of dry powder inhaler used is of significant importance to the efficiency of delivery of the active particles to the respiratory tract. Also, the physical properties of the active particles used affect both the efficiency and reproducibility of delivery of the active particles and the site of deposition in the respiratory tract.
On exit from the inhaler device, the active particles should form a physically and chemically stable aerocolloid which remains in suspension until it reaches an alveolar or other absorption site preferably in the lungs. Once at the absorption site, the active particle should be capable of efficient collection by the pulmonary mucosa with no active particles being exhaled from the absorption site.
The size of the active particles is particularly important. For effective delivery of active particles deep into the lungs, the active particles should be small, with an equivalent aerodynamic diameter substantially in the range of 1 to 5 xcexcm, approximately spherical and monodispersed in the respiratory tract. Small particles are, however, thermodynamically unstable due to their high surface area to volume ratio, which provides significant excess surface free energy and encourages particles to agglomerate. In the inhaler, agglomeration of small particles and adherence of particles to the walls of the inhaler are problems that result in the active particles leaving the inhaler as large agglomerates or being unable to leave the inhaler and remaining adhered to the interior of the inhaler.
The uncertainty as to the extent of agglomeration of the particles between each actuation of the inhaler and also between different inhalers and different batches of particles, leads to poor dose reproducibility. It has been found that powders are reproducibly fluidisable, and therefore reliably removable from an inhaler device, when the particles have a diameter greater than 90 xcexcm.
To give the most effective dry powder aerosol, therefore, the particles should be large while in the inhaler, but small when in the respiratory tract.
In an attempt to achieve that situation, one type of dry powder for use in dry powder inhalers may include carrier particles to which the fine active particles adhere whilst in the inhaler device, but which are dispersed from the surfaces of the carrier particles on inhalation into the respiratory tract to give a fine suspension. The carrier particles are often large particles greater than 90 xcexcm in diameter to give good flow properties as indicated above. Small particles with a diameter of less than 10 xcexcm may become coated on the wall of the delivery device and have poor flow and entrainment properties leading to poor dose uniformity.
The increased efficiency of redispersion of the fine active particles from the agglomerates or from the surfaces of carrier particles during inhalation is regarded as a critical step in improving the efficiency of the dry powder inhalers.
It is known that the surface properties of a carrier particle are important. The shape and texture of the carrier particle should be such as to give sufficient adhesion force to hold the active particles to the surface of the carrier particle during fabrication of the dry powder and in the delivery device before use, but that force of adhesion should be low enough to allow the dispersion of the active particles in the respiratory tract.
It is an object of the invention to provide a method of producing carrier particles for use in dry powder inhalers and to provide carrier particles that overcome or mitigate the problems referred to above.
According to the invention there is provided a method of producing particles suitable for use as carrier particles in dry powder inhalers, the method including the step of treating particles of a size suitable for use as carrier particles in dry powder inhalers to dislodge small grains from the surfaces of the particles, without substantially changing the size of the particles during the treatment.
The surface of the carrier particle is not smooth but has asperities and clefts in the surface. The site of a cleft or an asperity is often found to be an area of high surface energy. The active particles are preferentially attracted to and adhere most strongly to those high energy sites causing uneven and reduced deposition of the active particles on the carrier surface. If an active particle adheres to a high energy site, it is subjected to a greater adhesion force than a particle at lower energy sites on the carrier particle and will therefore be less likely to be able to leave the surface of the carrier particle and be dispersed in the respiratory tract. During the treatment asperities are removed as small grains, thus removing active sites associated with the asperities.
Advantageously, the small grains become reattached to the surfaces of the particles. The object of treating the carrier particles is to reduce the number of high energy sites on the carrier particle surfaces, thus allowing an even deposition of active particles adhered on the surface with a force of adhesion such that dispersion of the active particles during inhalation is efficient. While removing asperities as small grains removes those high energy sites associated with the asperities, the surfaces of the carrier particle have other high energy sites, for example at the site of clefts, which sites are not necessarily removed when the apserities are removed. It would therefore be highly advantageous to decrease the number of those high energy sites.
The grains removed from the surface are small and thermodynamically unstable and are attracted to and adhere to the high energy sites on the surface of the carrier particle. On introduction of the active particles, many of the high energy sites are already occupied, and the active particles therefore occupy the lower energy sites on the carrier particle surfaces. That results in the easier and more efficient release of the active particles in the airstream created on inhalation, thereby giving increased deposition of the active particles in the lungs.
Advantageously, the treatment step is a milling step. The milling process causes asperities on the surfaces of the carrier particles to be dislodged as small grains. Many of those small grains become reattached to the surfaces of the carrier particles at areas of high energy.
Preferably, the milling step is performed in a ball mill. Preferably, the carrier particles are milled using plastics or steel balls. Balls made of plastics material give less aggressive milling, whilst steel balls confer more efficient surface smoothing. Advantageously, the mill is rotated at a speed of less than about 60 revolutions per minute, more advantageously at a speed of less than about 20 revolutions per minute, and most preferably at a speed of about six revolutions per minute. That is a slow speed for ball milling and results in the gentle removal of grains from the surfaces of the particles and little fracture of particles. Fracture of the particles, which occurs with aggressive milling conditions, for example at higher milling speeds such as 60 revolutions per minute and/or long milling times, may result in agglomerates of fractured particles of carrier material. The use of agglomerates of particles as carrier particles has been found to lead to good deposition of active particles in the lower lung.
Advantageously, the particles are milled for at least one hour, preferably the particles are milled for about six hours. That time has been found to be suitable when milling with balls made from plastics material. When using denser balls, shorter milling times may be used. Alternatively, a different milling technique may be used, for example using a re-circulated low fluid energy mill, or other method that results in the removal of grains from the surfaces of the particles e.g. sieving.
The carrier particles may include may acceptable pharmacologically inert material or combination of materials. Advantageously, the carrier particles are crystalline sugar particles. Preferably, the carrier particles are lactose particles.
Advantageously, the diameter of the carrier particles lies between 50 xcexcm and 1000 xcexcm. Preferably, the diameter of the carrier particles is less than 355 xcexcm and lies between 60 xcexcm and 250 xcexcm, more preferably 90 xcexcm and 250 xcexcm. The relatively large diameter of the carrier particle improves the opportunity for active particles to become attached to carrier particles which is controlled by the above technique to provide good flow and entrainment characteristics and improved release of the active particles in the airways to increase deposition of the active particles in the lower lung.
The size of the carrier particles is an important factor in the efficiency of the inhaler, and an optimum, or near optimum, range of size of carrier particles is preferably selected. The optimum range of size of carrier particles may differ according to the inhaler device and active particles used. Thus, the method preferably includes the step of selecting an advantageous range of size of carrier particles prior to the treatment step. That step of selecting an advantageous range of size may be a sieving step.
According to the invention, there is also provided a method of producing a dry powder for use in dry powder inhalers, the method including the steps of treating carrier particles to dislodge small grains from the surfaces of the carrier particles without substantially changing the size of the carrier particles during the treatment step, and mixing the treated carrier particles with active particles such that active particles adhere to the surfaces of carrier particles.
Advantageously, the small grains become reattached to the surfaces of the carrier particles.
Advantageously, the method includes the steps of treating carrier particles according to the present invention and mixing the treated carrier particles with the active particles such that active particles adhere to the surfaces of carrier particles. The treatment of the carrier particles may be carried out before the active particles are added, but it may also be carried out in the presence of the active particles.
Advantageously, the carrier particles and the active particles are mixed in a container made from a plastics material. That has been found to give an unstable mixture of salbutamol and lactose and thus increases the deposition of salbutamol in the lungs. A container of different material may be used when using a mixture containing a different type of active particles.
Advantageously, the carrier particles and the active particles are mixed for at least five minutes. Preferably, the carrier particles and the active particles are mixed for about thirty minutes. The mixing should be for a time sufficient to give a homogeneous mixture of the active particles and the carrier particles, during mixing, rearrangement of the sites of particles may also occur, even when the system is homogeneous.
Advantageously, the mixing is interrupted and the mixture of carrier particles and active particles is sieved. The sieving of the mixture reduces the number of large agglomerates present. Preferably, the sieve mesh size is about 250 xcexcm.
The ratio in which the carrier particles and active particles are mixed is dependent on the inhaler device and the active particles used. For the example given below, a ratio of 125 to 1 by weight is preferably used.
Advantageously, the diameter of the active particles is between 0.1 xcexcm and 3 xcexcm such that the particles give a good suspension on redispersion from the carrier particles and are delivered deep into the respiratory tract.
The active particles may include a xcex22-agonist which may be terbutaline, a salt of terbutaline or a combination thereof or may be salbutamol, a salt of salbutamol or a combination thereof. Salbutamol and its salts are widely used in the treatment of respiratory disease. The active particles may be particles of salbutamol sulphate.
The active particles may include a steroid, which may be beclomethasone dipropionate. The active principle may include a cromone which may be sodium cromoglycate. The active principle may include a leukotriene receptor antagonist.
According to the invention, there are also provided particles suitable for use as carrier particles in a dry powder inhaler, the particles consisting of small grains and large particles to the surfaces of which the small grains are attached.
Preferably, the small grains have a diameter between 1 xcexcm and 5 xcexcm and, preferably, the large particles have a diameter between 50 xcexcm and 1000 xcexcm.
Preferably, the large particles are particles of lactose.
According to the invention, there are also provided particles suitable for use as carrier particles in a dry powder inhaler wherein the particles are made by a method according to the invention.
According to the invention there is further provided a dry powder suitable for use in a dry powder inhaler including carrier particles according to the invention and active particles, wherein active particles adhere to the surfaces of carrier particles.
The carrier particles usually consist of a particulate crystalline sugar. Lactose particles are often used as carrier particles.
The active particles referred to throughout the specification will be particles of one or a mixture of pharmaceutical products. Those pharmaceutical products include those products which are usually administered orally by inhalation for the treatment of disease such as respiratory disease eg. xcex2-agonists, salbutamol and its salts. Other pharmaceutical products which could be administered using a dry powder inhaler include peptides and polypeptides, such as insulin. In addition the method could find use in nasal delivery.
According to a further aspect of the invention, there is provided a method of producing particles including the step of treating large particles such that small grains adhere to the surfaces of the large particles.
As indicated above, the surfaces of the large particles are not completely smooth even following treatment such as milling but have asperities and clefts. As a result, the surfaces have areas of high surface energy to which active particles are preferentially attached. An active particle at a high energy site is less likely to be able to leave the surface and be dispersed in the respiratory tract than an active particle at a site of lower surface energy. During the treatment of the large particles, the small grains are attracted to and adhere to high energy sites on the surface of the large particles. On the introduction of the active particles, many of the high energy sites are already occupied, and the active particles therefore occupy the lower energy sites on the carrier particle surfaces. That results in the easier and more efficient release of the active particles in the airstream created on inhalation, thereby giving increased deposition of the active particles in the lungs.
Advantageously the step of treating large particles such that small grains adhere to the surfaces of the large particles is a mixing step. Small grains, or agglomerates of small grains, may be introduced to a sample of large particles which may have been treated to dislodge small grains from the surfaces of the particles and the mixture blended for several hours to allow the small grains to become attached to the surfaces of the large particles.
The small grains added to the large particles are preferably the product of milling large particles. If the large particles are subjected to aggressive milling, for example at high milling speed, small grains are produced. Those small grains may form larger agglomerates.
Advantageously, the large particles and small grains are mixed in a ratio by weight of at least one part of large particles to each part of small grains. For example, the proportion of small grains may be between 10 and 30 per cent, especially of the order of 20 per cent by weight based on the combined weight of the small grains and the large particles. It has been found to be highly advantageous for the surfaces of the large particles to become saturated with small grains. Some of the small grains may act as carrier particles for active particles, by leaving the surfaces of the large particles with active particles attached to their surfaces. The dimensions of the combined active particle and small grain are generally still within the optimum values for good deposition in the lower lung. It is believed that active particles which adhere to the small grains on the large particles, are preferentially released from the surface of the large particles.